1. Field of the Invention
This invention relates to a magnetic iron oxide and a process for preparing a magnetic powder which is useful for a magnetic recording medium.
2. Description of the Prior Art
In the field of magnetic recording technology, conventional magnetic powder such as .gamma.-Fe.sub.2 O.sub.3 have been found to possess unsatisfactory magnetic characteristics in view of recent improvements in magnetic recording systems. Because of these improvements, magnetic powder are needed which have a higher coercive force and a higher magnetic flux than obtainable with conventional magnetic compositions. One improved magnetic powder is an iron-nickel-cobalt alloy powder which was developed about ten years ago. However, it could not be practically used because of difficulties encountered in the preparation of the alloy. Another magnetic powder which has been developed is a cobalt modified .gamma.-Fe.sub.2 O.sub.3. It has been prepared by dispersing acicular iron oxide in a solution of cobalt chloride having a pH of 7.5 - 9.0, and then heating the dispersion at 80.degree.- 100.degree. C to form a mixture of cobalt hydroxide and acicular iron oxide. Thereafter, the solid material obtained was crushed and heated at 370.degree.- 425.degree. C to yield a cobalt modified .gamma.-Fe.sub.2 O.sub.3. However, in order to obtain magnetic powder characterized by high coercive force, relatively large quantities of the cobalt component had to be used. When a relatively large amount of the cobalt component is used, however, the thermal demagnetization and pressure demagnetization of the powder is increased and the degree of coagulation of the powder is increased so that the dispersibility in the preparation of magnetic paints is decreased.
The saturated magnetic flux density value of the cobalt containing composition decreases relative to increasing quantities of cobalt in the composition. Accordingly, magnetic recording media prepared with the magnetic powder have the following disadvantages with regard to their use in magnetic tapes:
1. The decrease in the residual magnetic flux density (Br) of magnetic tapes with increasing temperatures is high. Moreover, the demagnetization resulting from the increased temperatures and the subsequent cooling to room temperatures is also high. Heretofore, magnetic tapes have been susceptible to demagnetization caused by the heat generated by the friction between the magnetic tape and the magnetic head in the running of the tape. The demagnetization of the tapes, especially videotapes, which have a memory tape that passes at a relatively high rate of speed between the magnetic tape and the magnetic head, in the reproduction of magnetic recording signals is a particular difficulty which must be solved.
2. The decrease of the coercive force Hc of the magnetic tapes caused by an increse in temperature is high. The tendency increases when the coercive force Hc of the magnetic tape is high.
A need, therefore, continues to exist for the provision of a powdered magnetic composition which has a high coercive force and a high magnetic flux density, which has a high resistance to thermal demagnetization.